1. Field of the Invention
This invention relates to a magnetic bubble device. More particularly, the present invention relates to a magnetic bubble device which can propagate small magnetic bubbles of an extremely small diameter and is suitable for use in electronic computers, electronic switchboards, and so forth.
2. Description of the Prior Art
As is well known in the art, devices which are referred to as "permalloy devices" have been used as magnetic bubble devices in the past.
The devices of this kind are characterized in that a film of a soft magnetic material (permalloy) 1 having a plan such as that shown in FIG. 1, for example, is formed over a film of a magnetic garnet (not shown) capable of retaining magnetic bubbles, such as (YSmLuCa).sub.3 (FeGe).sub.5 O.sub.12 for example, thereby defining a magnetic bubble propagation track, and a rotating field is applied parallel to the garnet film so as to propagate the magnetic bubbles 2.
A generator, a transfer gate, a swap gate and a replicator for generating, transferring, swapping and replicating the magnetic bubbles, respectively, are formed by a conductor pattern 5 consisting of an Al-Cu or Au film, for example, as shown in FIG. 2 showing a section thereof. The conductor pattern 5 is sandwiched between the permalloy pattern (film) 1 and the magnetic garnet film 3 via insulating films 6 and 4, respectively. When a controlling pulse current is made to flow through the conductor pattern 5, various functions of the magnetic bubbles such as their generation or transfer can be executed.
Generally, a magnetic garnet film 3 which is capable of retaining magnetic bubbles is formed by epitaxial growth on the (111) plane of a substrate of a single non-magnetic crystal such as Gd.sub.3 Ga.sub.5 O.sub.12, but this substrate of a single non-magnetic crystal is omitted from the drawing for ease of comprehension because it does not pertain directly to the gist of the present invention.
The pattern width and gap dimensions of the permalloy propagation track 1 have been reduced markedly as the density as well as the degree of integration of the bubble device has increased. When fabricating a device having a bit period of 8 .mu.m using bubbles having a diameter of approximately 2 .mu.m, for example, the pattern and gap dimensions must be accurate to approximately 1 .mu.m. In order to further increase the density of devices in future by the use of permalloy devices, delicate permalloy patterns of less than 1 .mu.m must be formed accurately over the entire surface of the chip, this is extremely difficult technically.
A bubble device of a novel type has been drawing increasing attention in recent years (U.S. Pat. No. 3,828,329). In place of the propagation track consisting of a conventional soft magnetic material such as a permalloy, this prior art is characterized by including a propagation track which is formed by ion implantation and the device is referred to as an "ion-implanted device". In accordance with this prior art, the desired portion of the magnetic garnet film 3 is covered with a mask (not shown) having the shape of contiguous discs, as shown in FIG. 3, ions such as those of Ne, H.sub.2, etc., are implanted into the surface of the magnetic garnet film so that an ion-implanted region 7 is defined in the exposed portion outside the mask with the contiguous disc shape, and the magnetization of the region 7 is directed parallel to the film plane. When a rotating field is applied parallel to the garnet film, bubbles can be transferred along the edges of the contiguous discs (propagation track) 8 in the same way as in a conventional permalloy device. This ion-implanted device has an essential feature in that the pattern dimensions of the propagation track 8 can be about twice that of the propagation track of a permalloy device, so that the device can be fabricated easily and is suitable for a high density device.
The critical problems for the practical application of this ion-implanted device are that the functions of the replicator, transfer gate, swap gate, etc., are not sufficiently stable, a block replicator can not be formed easily and the device can not operate at high speed.
To eliminate these problems, a magnetic bubble memory device has been proposed (U.S. Patent Application Ser. No. 375,344, filed May 5, 1982 and now U.S. Pat. No. 4,528,645) in which a minor loop consists of a propagation track formed by ion implantation, and at least part of a major loop (or major line) as described above, as well as at least part of the junction between the minor loop and the major loop are formed by the use of a permalloy film.
For convenience, the device having such a structure will be hereinafter referred to as a "hybrid bubble memory device": In this device, the minor loop that accounts for the larger part of the area of the device is formed by ion implantation suitable for a high density memory, while part of each of the major loop (or major line), replicator, transfer gate, detector and generator is formed of a film of a soft magnetic material such as a permalloy. This arrangement makes it extremely easy to fabricate a minor loop for transferring small magnetic bubbles, to ensure the stable operations of the replicator, transfer gate, swap gate, etc., that have not been accomplished by conventional ion-implanted devices, and also to fabricate the block replicator easily.
Since this hybrid bubble memory device includes a propagation track formed by ion implantation and a propagation track consisting of a soft magnetic film, however, it is not free from the problem that when magnetic bubbles pass through the junction between the tracks, the propagation margin of the magnetic bubbles drops markedly. This problem has to be solved before the hybrid bubble memory device can be put to practical use.